Normally, this file served as plain text. This can be seen when clicking on the link and opening the Page information dialog in Firefox (Ctrl+I). While plain text may be fine in another context, it is required that the Content-Type header is set to application/x-web-app-manifest+json. More precisely, this is required for the App submission on the Firefox Marketplace. It is recommended on MDN to tune the Apache server configuration with a .htaccess file. However, with my shared hosting plan at OVH, this was not working: the .htaccess file had no effect. [Edit: it seems to work now, so this PHP trick may not be needed anymore !].

To bypass this hosting limitation, I tried with a snippet of PHP code that simply sourcing the original manifest.webapp file, but with the proper Content-Type header.